1. Field of the Invention
This invention relates to a precise method to accurately measure the color of three dimensional non-specifically oriented objects and an associated apparatus.
2. Description of the Related Art
Color is a characteristic of human sight that is based on the wavelength composition of light which is reflected from an object to the eyes. The light is measured photochemically by small sensors on the retina of the eyes. The eye has some receptors or sensors which are rods and others which are cones. The cones mediate all color vision. These cones are divided in to three basic categories, each responsive to distinctive overlapping portions of the visible light spectrum. These three categories of sensors are referred to by their dominant wavelength sensitivities in the red, green, and blue portions of the spectrum. The simultaneous perception of a triad of sensors (tristimulus) causes humans to perceive colors for which there is no corresponding monochromatic wavelength. Variations in an individual's rods and cones may cause a given color to be perceived differently from person to person. In order to allow colors to be analyzed accurately, it is necessary to have a system which accurately and reliably determines object color.
It is important in color measurement to have a good light source that facilitates effective color determination.
Only a small percentage of humans possess the color acuity for discriminating slight color variations. Instruments have historically employed a very diffuse lighting when measuring color in order that no directional source of light can affect the apparent color of an object. It has been known to attempt to duplicate human color vision in automated equipment.
There have been two principal strategies for measuring color of an object by apparatus other than the human eye. One approach is to measure the ratio of reflected light to incident light with respect to the object for each wavelength, or for many groups of wavelengths, in the visible spectrum. The resulting reflectance data is unique for each color. Instruments of this type, however, do not exhibit the same sensitivity response to colors as humans and, therefore, are of limited use in determining how closely a human will perceive the difference between two colors. The other measuring strategy attempts to mimic human sight by employing tristimulus colors similar to the red, green, and blue sensors in human eyes. Standard color systems have been devised which reflect the average responses of many people. A video camera is an example of such a standard color system.
A limitation common to human sight and video camera sensitivity alike is that some colors appear the same in one light but different in another. Because only three sensors are employed, different wavelengths of light within a sensor's band of sensitivity stimulate the sensor similarly.
It has been known to measure color by obtaining a flat homogeneous sample of the colored material and presenting it to a port of, for example, a spectrophotometer. Spectrophotometers typically employ diffuse lighting and measure an average color of the object viewed through the entire field. Spectrophotometers directly measure the reflectance spectrum of a two dimensional object and compare it with the incident spectrum. This apparatus is not suitable for use in determining surface color on a complex, non-planar geometric surface.
There remains, therefore, a need for a system to provide accurate color measurement of a non-planar surface and more particularly there is need for such a system which does not require particular surface orientation.